The present invention relates to pumps, particularly for membrane filtration systems such as desalination systems, and to such systems including the pumps.
In a typical desalination system using a reverse osmosis membrane, a saline solution is supplied to a membrane unit at high pressure, for example between 650 and 950 Psi (4500 to 6500 kPa). After passing through the saline side of the membrane unit the volume of solution is reduced and the salinity is increased—with a portion of the fresh water extracted through the membrane. This exhaust liquid or concentrate is still at a high pressure.
The saline solution for feeding to the membrane unit is pressurised by a feed pump.
In some systems the feed pump may have a single cylinder, and may be manually operated by a lever.
In larger systems the feed pump may be mechanically driven, for example by an electric motor, and have multiple cylinders.
U.S. Pat. No. 4,434,056 describes two examples of a feed pump which includes energy recovery. The first example has 3 cylinders. The second example has 5 cylinders. In both examples the valving for energy recovery from the exhaust liquid is integrally provided as a spool valve arrangement on the piston rods. The valving for each cylinder is provided on the piston rod of another cylinder. Both examples include complex pistons with integrated lost motion mechanisms to provide a dwell time at each end of the piston rod stroke. This dwell time accommodates the angular period for which the valving for the respective piston is closed.
The inclusion of a lost motion mechanism in the piston has several problems. The mechanism is complex and has parts which collide in use, necessitating buffers. In addition operation of the dwell mechanism can be adversely effected by pre-pressurised supply of the feed liquid, as the dwell effect needs a negative pressure on the supply side of the piston.
In this specification, where reference has been made to external sources of information, including patent specifications and other documents, this is generally for the purpose of providing a context for discussing the features of the present invention. Unless stated otherwise, reference to such sources of information is not to be construed, in any jurisdiction, as an admission that such sources of information are prior art or form part of the common general knowledge in the art.
For the purpose of this specification, where method steps are described in sequence, the sequence does not necessarily mean that the steps are to be chronologically ordered in that sequence, unless there is no other logical manner of interpreting the sequence.